Customizing of the app-sync name

[mwawrusch]

It seems that the name of the instance as specified in the serverless.yml is also used as the name withing app sync, plus some unique id. It would be preferable if that could be overwritten, in our case I would prefer the name to include the stage to make identification easier.

E.g. issuer becomes issuer-prod-234drfs3 when deployed for production.